Oil sand, also referred to as tar sand or bituminous sand, is a combination of clay, sand, water and bitumen that is mined below the surface of the earth. Oil sand deposits are found all over the world, the largest deposits being located in Venezuela and Alberta, Canada. On average, the bitumen content of these oil sands ranges from about 5 to 20 percent by weight or more of the total weight of oil sand. Thus, oil sands represent a rich source of heavy oil.
Surprisingly, oil sands, such as mined in Athabasca, Alberta, also contain heavy minerals such as zircon, ilmenite, rutile and leucoxene, which heavy minerals are concentrated and upgraded during processes used to extract the bitumen from oil sands. Thus, there have been a number of attempts to recover these heavy minerals from oil sand tailings that are produced after the majority of the bitumen has been extracted from the oil sands.
Several processes have been developed throughout the years for extracting bitumen from oil sands. Oil sand lends itself to liberation of the sand grains from the bitumen, preferably by slurrying the oil sand in hot process water, thereby allowing the bitumen to move to the aqueous phase. For many years, the bitumen in the Athabasca sand has been commercially removed from oil sand using what is commonly referred to in the industry as the “Clark Hot Water Process”.
In general terms, the Clark Hot Water Process involves dry mining the oil sand and feeding the oil sand into a rotating tumbler where it is mixed for a prescribed retention time with hot water (approximately 80-90° C.), steam, process aids (e.g., sodium hydroxide) and naturally entrained air to yield an oil sand slurry. During slurrying, the bitumen matrix is heated and becomes less viscous, chunks of oil sand are ablated or disintegrated and the released sand grains and separated bitumen droplets are dispersed in the water to produce “conditioned” oil sand slurry.
In recent years, lower temperature extraction processes have been developed to conserve energy. For example, a “warm slurry extraction process” has been developed and is disclosed in Canadian Patent No. 2,015,784. In the warm slurry extraction process oil sand is mixed with sufficient hot water to yield an oil sand slurry having a temperature in the range of 40-55° C. Mixing and conditioning occurs in a tumbler, with retention times being increased to within the range of about 7-12 minutes to produce conditioned slurry. In the alternative, conditioning can occur in a pipeline, commonly referred to as “pipe-line conditioning”, as disclosed in Canadian Patent No. 2,029,795 and U.S. Pat. No. 5,039,227.
In the late 1990s a cold dense slurrying process for extracting bitumen from oil sand was developed, which is disclosed in Canadian Patent No. 2,217,623 and U.S. Pat. No. 6,007,708. This process is commonly referred to as the “low energy extraction process” or the “LEE process” and involves mixing the mined oil sand with water in predetermined proportions near the mine site to produce a slurry containing entrained air and having a controlled density in the range of 1.4 to 1.65 g/cc and preferably a temperature in the range 20-40° C. The slurry is then pumped through a pipeline having a plurality of pumps spaced along its length, preferably adding air to the slurry as it moves through the pipeline, to condition the slurry.
The oil sand slurry produced by any of the extraction processes described above is typically diluted with additional water and the diluted slurry is introduced into a primary separation vessel (PSV) where the more buoyant aerated bitumen rises to the surface and forms a froth layer. The bitumen froth so produced is commonly referred to as “primary froth” and typically comprises 65 wt. % bitumen, 28 wt. % water and 7 wt. % solids.
Typically the PSV underflow and middlings are further treated in secondary treatment vessels such as flotation vessels or gravity separators to produce a “secondary froth” typically comprising 45 wt. % bitumen, 45 wt. % water and 10 wt. % solids. The primary and secondary froths are then pooled together for further processing.
There are other water extraction processes, such as the known OSLO process, the Bitmin process, and the Kryer process, which also produce bitumen froth that can be further processed to produce bitumen and oil sand tailings that are useful in the present invention.
The bitumen in the bitumen froth is separated from the water and solids enriched in heavy minerals by first diluting the froth with sufficient solvent or diluent such as naphtha and then subjecting the diluted froth to further processing to first remove coarse solids and then remove fines. For example, a plurality of scroll-type centrifuges could be used or a combination of scroll separators/centrifuges and disc separators/centrifuges can be used as described in Canadian Patent No. 1,072,473. It is understood by those skilled in the art that other processes can separate the bitumen from the water and solids, for example, counter-current decantation, or treatment using inclined plate settlers or stationary froth treatment, or by cyclones or some combination of these. The resultant “cleaned” bitumen is further refined into useful products and the remaining water and solids (referred to in the industry as “oil sand tailings”) are disposed of in tailings ponds.
The oil sand tailings obtained from centrifugation operations and the like typically contain 16-20% solids by weight, 76-80% water by weight, and from about 2% to about 5% residual bitumen. The solids portion on average comprises 15-30 Wt. % or more heavy minerals, of which typically 30-70 wt. % consists of valuable minerals such as zircon, minerals containing titanium dioxide (e.g., ilmenite, leucoxene and rutile) and the like. The remainder of the solids, approximately 70-85% by weight, is generally comprised of fine silica, mica, petrified wood chips and fine clays. It is this oil sand tailings material that provides the feedstock suitable for use in the process of the present invention.
Because of the unique composition of the oil sand tailings obtained from oil sand extraction and bitumen froth treatment, there are many challenges one faces when attempting to separate out heavy minerals therein. The four main problem areas that have created difficulties in recovering heavy minerals from tailings, and have made it difficult to use normal mineral dressing techniques of the industry, are as follows:                1. the bitumen coating on the sand grains;        2. secondary mineralization such as pyrite, siderite and calcite;        3. a high proportion of alumino-silicates (approximately 25 percent) that have similar gravity, electrostatic and magnetic properties as other minerals in the heavy mineral suite; and        4. the presence of slimes or fines (fine particulates smaller than 45 μm in diameter) that varies between about 25-40% by weight.        
Several attempts have been made to deal with the problem of residual bitumen. For example, Canadian Patent Nos. 1,076,504 and 1,088,883, describe processes in which the residual bitumen is burned off prior to the separation of the heavy minerals. However, the high temperatures used will alter the properties of the heavy minerals in such a manner that magnetic and/or electrostatic processes are no longer efficient in selectively separating out the minerals.
Canadian Patent No. 1,326,571 discloses subjecting bitumen-containing tailings obtained directly from centrifugation of bitumen froth to a high pH (from about 8 to about 11.5) flotation step in a flotation vessel by aerating the tailings, which causes the mineral values to float in a froth above the siliceous and other unwanted material. The removed froth solids are then preferably heated to burn off residual bitumen.
However, the flotation process described in Canadian Patent No. 1,326,571 is not very efficient in separating out heavy minerals for further processing due to the high concentration of bitumen present during the flotation step (i.e., from about 2 to about 5 wt. %) and the presence of fines or slimes. Furthermore, the heavy mineral froth obtained from flotation still contains a large amount of bitumen, which must subsequently be removed before further processing can occur.
It has now been discovered that the addition of (1) a desliming step, to remove a substantial amount of slimes or fines (i.e., solids less than 45 μm) and residual bitumen, and (2) an attritioning step, to remove additional fines and residual bitumen, prior to a step of separating and/or concentrating heavy minerals from the siliceous material, such as in a flotation cell or gravity concentrator, greatly improves overall recoverability of heavy minerals from oil sand tailings, and provides a concentrated heavy mineral feed material from which zircon, ilmenite, leucoxene, rutile, and the like can be separated and recovered.